1. Field of the Invention
This invention relates to a digital communication network in which digitized voice signals and data signals are treated equally or simultaneously, and more particularly to a voice signal converting device for use in such a digital communication network which can assure bit integrity of the data signals and guarantee speech quality of the voice signal.
2. Description of the Prior Art
Telephone networks set up primarily for voice communication have gained their extensive service area and accordingly have had many applications in non-telephone service communications such as a data communication and a facsimile. In particular, currently developed digitization in telephone communication networks promotes the advent of a digital link of the end to end type which makes way for availability of a data communication in the form of a digital voice channel of, for example, a 64 Kb/s rate. This rate is highly beneficial as compared with a maximum rate of 9.6 Kb/s resulting from a data communication using an existing analog voice channel of 4 KHz, and desirability has been concentrated to its realization.
However, in order to accomplish a digital data communication using the digital link, it is necessary to assure bit integrity in which a bit strip of digital data is conserved within a telephone communication network.
On the other hand, desired speech quality of voice communications is sometimes out of guarantee within the network in which bit integrity is realized. For example, if differently coded systems, respectively based on PCM .mu.-law and PCM A-law which are widely used as coding laws for voice signals in existing telephone networks, are directly connected, speech quality is disadvantageously degraded. In other words, when a subscriber in an area using the .mu.-law communicates with a subscriber in an area using the A-law, signals encoded and digitized pursuant to the .mu.-law are decoded into analog signals pursuant to the A-law within the network meeting bit integrity with a result that the original voice cannot be restored, thus imposing fatal inconveniences upon the communication.
A countermeasure for this problem is to provide a device for .mu.-A law conversion at a proper site within the network (generally at an international gate office). In addition to necessity of the .mu.-A law conversion, for guarantee of speech quality, converting devices for the voice signal such as an echo suppressor, a digital PAD and a band compressor are required which are necessarily inserted in the digital link.
Accordingly, when it is desired to equally or simultaneously treat both the digitized voice signal and data signal within a digital communication network, the use of a net voice channel in the data communication is prone to inconveniences and an expedient to solve this problem is required.
It is conceivable that an approach to the above problem is to select a digital link corresponding to the type of calls on the basis of call classification, namely, corresponding to either telephone call or data call and then to insert a necessary converting device in the link corresponding to the telephone call. However, this approach suffers from inconvenience that dynamic switching between voice and data during communication cannot be effected. In other words, universal versatility in services is important in a digital communication network and when a telephone terminal and a data terminal are connected through a single digital subscriber line for transmission and reception of digital voice signal and data signal between terminal units, it is desirable to realize dynamic switching between voice and data during communication. The above desirability, however, cannot be attained by either the insertion/removal or the activation/deactivation of the .mu.-A law converting device in accordance with the type of call and the terminal unit or the area to be communicated.
A conceivable approach to this problem is such that during communication, a subscriber handles to issue to an exchange a signal for switching between voice communication and data communication, and this switching signal is carried about on a common line signal network so as to control a voice signal converting device at an international gate office. However, this expedient is disadvantageous in that a controlling process is required of the exchange each time switching between voice communication and data communication is effected, resulting in excessive load on the exchange while the voice/data switching governed by the subscriber is difficult in synchronization with the controlling of the voice signal converting device.